This invention relates to a side pincushion correction circuit for use in display systems such as television systems.
It is well recognized that the geometrical relationship of the scanning beam or beams of a cathode ray tube to the inside surface of the viewing screen over which they are scanned causes a raster distortion known as pincushion distortion. This distortion is characterized by a widening of the display in a horizontal direction at the top and bottom of the raster relative to the center of the raster. It is also well recognized that in many situations this distortion can be corrected by modulating the horizontal scanning current at the vertical deflection scanning rate. Frequently the modulation is in parabolic form, causing a maximum decrease in horizontal scanning current at the top and bottom of the raster, corresponding to the beginning and end portions of the vertical trace interval, relative to the horizontal scanning current at the center of the raster.
The vertical rate modulation may be impressed upon the horizontal scanning current in a number of ways. A reactor may be placed in circuit with the horizontal deflection coils and controlled to vary the impedance to horizontal scanning current at the vertical rate. A transformer in circuit with the horizontal deflection coils may also be utilized to achieve the required modulation. A more recent approach is to rectify horizontal rate energy, such as that obtained from a winding of the horizontal output transformer, and utilize the rectified current for supplying the vertical deflection output stage. The vertical stage itself then loads the horizontal winding such that the horizontal scanning current is modulated at the vertical deflection rate. While this arrangement is satisfactory, it would be desirable to provide a raster correction circuit which offers a greater degree of control over the modulation.
In accordance with the invention, a raster correction circuit includes a line frequency generator and a line frequency output transformer and deflection winding coupled to the line frequency generator. Power supply means including rectifying means are coupled to a winding of the transformer for rectifying line rate energy obtained from the winding. A field rate generator is coupled to the power supply means for obtaining operating current therefrom. Active current conductive means are coupled in circuit with the rectifying means and are responsive to signals derived from the field frequency generator for providing a variable impedance path for varying the current in the rectifying means at a field deflection rate for loading the transformer and varying line rate current in the deflection winding in a manner to correct for raster distortion.